Quick answer: start with 50 W of solar
Almost nothing, in solar terms. A 90 W pump that runs about 30 minutes a day uses roughly 45 Wh, so the smallest 50 W planning array on this site covers it with room to spare. The real design work is the DC circuit: the right fuse, a cable heavy enough for the run, and a pressure switch that does not short-cycle.
Planning result
At 200 W per module, the array target is 1 panel. That count assumes four peak-sun-hours, 75% end-to-end solar yield, 90% conversion efficiency, and a 20% planning margin (conservative here, because a DC load has no inverter in its path). Seasonality is small for this load; freeze protection for the lines, pump, and tank is the bigger winter concern.
How a 12V water pump actually uses power
Planning case: a 12 V on-demand diaphragm pump of the kind used in RVs, vans, boats, and tank-fed cabins, drawing about 90 W (7.0 A) while a tap is open, for about 30 minutes of pumping a day. SHURflo's data sheet for its 4008 pump shows 2.2 A at open flow, 6.0 to 6.8 A at 40 to 50 psi, and a 7.5 A maximum, with a 10 A fuse recommended.
The pump runs only while water flows. Its pressure switch starts it when line pressure falls to the cut-in setting and stops it at cut-out, so short draws such as hand-washing start and stop it many times while a shower keeps it running continuously. Daily energy is set by the water you use, not by how long the system is switched on.
Load calculation from running watts and duty cycle
The appliance-class estimate is 90 W running for 0.5 h/day at a 100% duty cycle, an average of about 90 W across those hours:
Then losses and reserve raise the energy the system must collect to about 60 Wh/day.
Typical manufacturer nameplate/manual range for the 12V water pump class, triangulated with SHURflo (Pentair): Revolution 4008 RV fresh water pump technical data sheet and SHURflo (Pentair): Revolution 4008 series by-pass pump, then reduced to a conservative planning midpoint; this is not a product specification. Verify the exact unit's nameplate and manual before sizing.
Measure with a DC clamp meter or the battery monitor's shunt: note the amps with one tap open and time a typical day of use. The maximum current on the label occurs near the pump's top pressure, so real draw is usually lower.
Solar and battery sizing table
| Design item | Planning value | Basis |
|---|---|---|
| Appliance energy | 45 Wh/day | Running W × hours × duty cycle |
| Solar collection target | 50 W | 4 peak-sun-hours, 75% yield, 20% margin |
| 200 W panel count | 1 | Rounded up; real modules must satisfy controller voltage/current limits |
| One-day battery allowance | 200 Wh | 90% conversion efficiency, 80% LiFePO4 usable fraction, 20% margin |
| 12.8 V LiFePO4 equivalent | 20 Ah | Nominal planning conversion |
| 25.6 V LiFePO4 equivalent | 10 Ah | Nominal planning conversion |
The 200 Wh minimum battery allowance on this site is far more than one pump needs, and that is the point: a pump like this is usually one small load on a battery that already serves lights, fans, and a 12 V refrigerator.
Build the closest passing wiring-diagram preset
The 12V Water Pump Planning System preset is an engine-passing starting point whose array, battery bank, continuous inverter rating, and surge rating each meet or exceed this page's stated planning allowances. Adjust it only from measured load data and verified equipment limits.
Inverter or DC branch circuit?
This pump runs directly from the battery, so no inverter is involved. Its permanent-magnet DC motor draws a short pulse above running current each time the pressure switch closes; that pulse sets the fuse rating, not the energy budget.
On a 12.8 V LiFePO4 bank the 90 W running estimate is about 7.0 A, with about 10.2 A for the brief start. Size the branch fuse to the manufacturer's instruction and the conductor to that fuse, then check voltage drop over the real round-trip length with the wire size calculator. This page still records a 200 W / 200 W inverter allowance for the diagram preset; it only matters if you choose a 120 V version of this load.
The diagram preset still includes an inverter because the engine models a complete system that can also serve AC loads. This appliance belongs on its own fused DC branch from the battery or DC distribution block, not on the inverter output.
The common mistake
Ways to cut the load
- Add a small accumulator tank so short draws do not start the pump every time.
- Fix drips and running toilets first; a leaking fitting makes the pump cycle all night.
- Use a low-flow shower head, because shower minutes are most of the pump's daily runtime.
- Mount the pump close to the tank and the battery to keep both the suction line and the DC run short.
What to verify before buying equipment
- The exact running watts, startup current, voltage, frequency, and required circuit from the appliance label and manual.
- Actual watt-hours over a representative day; seasonal heating or cooling loads need a design-day measurement.
- Local peak-sun-hours for the worst operating season, plus shade and array orientation.
- Controller maximum input voltage at the coldest expected temperature and maximum output current.
- Battery discharge current, series/parallel permission, terminal torque, low-temperature controls, and overcurrent protection.
- Conductor ampacity, voltage drop, terminal temperature ratings, disconnecting means, grounding, local amendments, and inspection requirements.
For a whole-system load list, use the load and appliance calculator. For the system context around this appliance, continue to the Solar water and well-pump guide.
Frequently asked questions
How much solar do I need for a 12V water pump?
For the planning case on this page (a 12 V diaphragm pressure pump running about 30 minutes a day), allow about 50 watts of solar at four peak-sun-hours per day. That covers an estimated 45 Wh daily load plus conversion, temperature, and recharge margin. The pump is rarely the reason to add panels; size the array for the rest of the DC loads.
Does a 12V water pump need an inverter?
No. A 12 V pump connects to a fused DC branch from the battery or DC distribution panel. It draws about 7.0 A at 12.8 V while running, so the fuse and wire are sized from that figure and the manufacturer's instructions; an inverter matters only if you buy a 120 V pump instead.
What battery size keeps a 12V water pump running for one day?
The one-day planning allowance is about 200 Wh nominal usable design capacity, roughly 20 Ah at 12.8 V or 10 Ah at 25.6 V for LiFePO4. In practice the pump shares a battery with the other DC loads and adds only a small slice of its daily budget.